Scientists who carried out a study of heat loss from a typical 1930s semi-detached home to measure the impact of retrofit insulation have found that current estimates of energy performance may be underestimating the heat lost
A paper published in the SCI journal Energy Science and Engineering titled ‘Whole‐House Thermal Bridge Modelling for Accurate Retrofit Energy Prediction’ says that seemingly small weak spots in a home’s insulation create a route for heat to escape.
These crucial gaps, known as thermal bridges, occur around walls, floors, roofs, and windows and can significantly affect how much heat is lost, even after homes are insulated.
Retrofit reduces heat transfer by over 60%
Researchers from the University of Salford and Leeds Beckett University found that a carefully designed whole-house retrofit reduced the home’s overall heat transfer coefficient by up to 63% with external wall insulation and 61% with internal wall insulation, compared with the original building.
However, they also warn that commonly used default figures for estimating heat loss are often not representative, particularly after homes have undergone piecemeal insulation retrofits.
This matters because energy models predict the effects of improvements to Britain’s housing stock and inform frameworks including Energy Performance Certificates and retrofit assessments.
Better attention needs to be paid to thermal bridges
Dr Christopher Tsang, lead author of the study, from the University of Salford, explained: “Retrofitting insulation into homes has become increasingly important to help meet climate targets. However, as buildings become better insulated, thermal bridges can account for a larger share of heat loss.
He added: “Rather than simply asking how much insulation should be installed, more attention needs to be paid to how insulation joins together across a whole building to fill in the thermal bridges.
“As well as heat loss, poorly addressed thermal bridges can also produce colder internal surfaces, which in turn can increase the risk of condensation and mould. We found that whole-house approaches could help mitigate surface-condensation risks while reducing overall heat loss.”
Better data needed on heat loss from insulated homes
Dr Tsang stated: “Our call to government and industry is that we need better data on where heat escapes from different types of homes. Using more realistic figures could improve predictions of how well insulation and other retrofit work will actually perform.
“Assessments should also take better account of the benefits of improving the whole house together, rather than treating measures such as wall, roof and floor insulation separately.”
The study draws particular attention to the simplified assumptions used when assessing existing houses.
Under the Reduced Data Standard Assessment Procedure, or RdSAP, bespoke calculations of thermal bridge heat loss are not normally used. Instead, a default value is applied across a property. The research notes that the default applies regardless of factors such as the property’s age, type, and insulation level.
While that simplification may have practical advantages, modelling every building junction individually would be costly and time-consuming, and the precise construction of an older home is often unknown. But the researchers argue that generic assumptions matter more when modelling the effects of fabric retrofits.
Their proposed alternative is not necessarily bespoke computer modelling for every British home. Instead, the study calls for a more comprehensive and representative database of thermal bridge values covering common building types and PAS 2035-compliant retrofit scenarios, which could make energy-performance predictions more reliable.
